Multi-phase DC/DC converters are generally used for high current applications, such as for generating a load current of 100 A or more. Employing multiple phases of the converter allows a sharing of the current generation, lowers ripple, and achieves other well known benefits. There may be two or more phases. Each phase should be identical for optimal operation, but this is impossible to achieve due to practical considerations.
For a multi-phase current mode DC/DC converter, each phase generally comprises a pulse width modulation (PWM) comparator, an RS flip flop for turning a transistor switch on and off at a certain duty cycle to achieve regulation, an inductor, and a current sense device such as a low value resistor. The turn-on time for each phase is controlled by a phased clock. When a switch turns on, a ramping current flows through its associated inductor. The phased ramping currents through the inductors are filtered by an output capacitor common to all phases. An error amplifier compares the output voltage of the converter to a reference voltage and generates a control voltage whose level corresponds to the duty cycle of the phases needed to maintain the output voltage at the desired voltage. The PMW comparator for each phase then compares the control voltage level to the current ramp through the inductor. When they cross, the RS flip flop controls the switch to turn off. In this manner, ideally, each phase is controlled to have the same duty cycle and contributes an equal amount of current to the load.
The temperatures of the various phases are slightly different due to at least the following factors: 1) the different physical positions of the various phases on a circuit board; 2) unequal cooling of the phases by forced air flow and heat sinking; and 3) non-matching components. This difference in operating temperatures causes the electrical characteristics of the phases (e.g., the on-resistances of MOSFET switches, the inductances, the current sense resistances, etc.) to be affected differently, resulting in non-matching currents being supplied at the same duty cycles and increased ripple. Such temperature imbalances may also result in “hot spots” that reduce the reliability and performance of the system.
This problem applies to all types of multi-phase DC/DC converters that use a sensed current in the feedback loop for regulation.
What is needed is a technique for balancing the temperatures of the various phases in a multi-phase DC/DC converter.